I work in XHDPI (closest to Retina res, so assets from iOS design can easily be pulled over to Android) in Sketch and export assets for MDPI, HDPI, XHDPI, XXHDPI. I usually work closely with the dev team, so I don't deliver a spec, but rather talk to them. Lately we have begun using Zeplin (which is soon out of Beta), so they can inspect all sizes and colors, fonts etc.